This invention relates to integrated circuits, and more particularly, to ways in which to improve the performance of metal-oxide-semiconductor transistor circuits on integrated circuits by cross-linking the gates and bodies of certain transistors.
Integrated circuits based on metal-oxide-semiconductor (MOS) transistor technology are widely used in modern electronic systems. As device dimensions shrink due to improvements in process technology, silicon-on-insulator (SOI) substrates are being considered for future generations of MOS integrated circuits.
MOS transistors such as the MOS transistors found on SOI substrates have four terminals: a source, a drain, a gate, and a body (sometimes also called the substrate). If the body terminal of an MOS transistor is allowed to float, the signals on the other terminals of the transistor will affect the body.
The voltage on the body, in turn, affects the electrical properties of the transistor. A low body voltage serves to increase the threshold voltage of the transistor and a high body voltage serves to decrease the threshold voltage. If the body voltage is low, the transistor will be harder to turn on and will therefore switch more slowly. If the body voltage is high, the transistor will switch more rapidly.
Because the voltages on the gate, source, and drain terminals of a transistor influence the body voltage and therefore the threshold voltage and switching speed of the transistor, transistors with floating bodies tend to experience hysteresis. Different types of pulses and different patterns of pulses will propagate through a circuit made from MOS transistors with floating bodies differently. The dependence of the electrical behavior of a MOS transistor with a floating body on its environment can be difficult to model and can make device performance unpredictable.
The performance of an MOS transistor can be made more predictable by electrically connecting the body to a source of known potential (i.e., to ground). Such “tied-body” configurations offer predictable performance, because the body is maintained at a known constant voltage, regardless of the signals applied to the gate, source, and drain terminals.
However, tied-body configurations in which the body is grounded tend to have high threshold voltages. The body contact diffusions and contact pads used for tied-body transistors also tend to consume a fair amount of real estate and may add undesirable complexity to the fabrication process.
It is an object of the present invention to provide improved MOS transistor arrangements that overcome some of the problems associated with floating body and tied-body structures.